Various exhaust after-treatment devices, such as particulate filters and other devices, have been developed to effectively limit exhaust emissions from internal combustion engines. In the case of compression-ignition or diesel engines, a great deal of effort continues to be expended to develop practical and efficient devices and methods to reduce emissions of largely carbonaceous particulates otherwise present in the engine's exhaust gas.
An after-treatment system for a modern diesel engine exhaust typically incorporates a diesel oxidation catalyst (DOC) as one of the devices for such a purpose. A DOC generally contains precious metals, such as platinum and/or palladium, which serve as catalysts to oxidize hydrocarbons and carbon monoxide present in the exhaust flow into carbon dioxide and water. A significant amount of an engine's hydrocarbon emissions may however burn on the DOC and cause elevated temperatures and eventual damage to the catalyst.
Additionally, an after-treatment system may also incorporate a diesel particulate filter (DPF) for collecting and disposing of the sooty particulate matter emitted by the diesel engine prior to the exhaust gas being discharged to the atmosphere. A typical DPF acts as a trap for removing the particulate matter from the exhaust stream. Similar to a DOC, the DPF contains precious metals, such as platinum and/or palladium, which serve as catalysts to further oxidize soot and hydrocarbons present in the exhaust stream. The DPF may be regenerated or cleaned using superheated exhaust gas to burn off the collected particulate.